Efficient induction of CCR9 on T cells requires coactivation of retinoic acid receptors and retinoid X receptors (RXRs): exaggerated T Cell homing to the intestine by RXR activation with organotins

SWI/SNF chromatin remodeling factor BAF60b restrains inflammatory diseases by affecting regulatory T cell migration

Regulatory T (Treg) cells play a critical regulatory role in the immune system by suppressing excessive immune responses and maintaining immune balance. The effective migration of Treg cells is crucial for controlling the development and progression of inflammatory diseases. However, the mechanisms responsible for directing Treg cells into the inflammatory tissue remain incompletely elucidated. In this study, we identified BAF60b, a subunit of switch/sucrose nonfermentable (SWI/SNF) chromatin remodeling complexes, as a positive regulator of Treg cell migration that inhibits the progression of inflammation in experimental autoimmune encephalomyelitis (EAE) and colitis animal models. Mechanistically, transcriptome and genome-wide chromatin-landscaped analyses demonstrated that BAF60b interacts with the transcription factor RUNX1 to promote the expression of CCR9 on Treg cells, which in turn affects their ability to migrate to inflammatory tissues. Our work provides insights into the essential role of BAF60b in regulating Treg cell migration and its impact on inflammatory diseases.

Retinoid X Receptor agonists as selective modulators of the immune system for the treatment of cancer

Upon heterodimerizing with other nuclear receptors, retinoid X receptors (RXR) act as ligand-dependent transcription factors, regulating transcription of critical signaling pathways that impact numerous hallmarks of cancer. By controlling both inflammation and immune responses, ligands that activate RXR can modulate the tumor microenvironment. Several small molecule agonists of these essential receptors have been synthesized. Historically, RXR agonists were tested for inhibition of growth in cancer cells, but more recent drug discovery programs screen new molecules for inhibition of inflammation or activation of immune cells. Bexarotene is the first successful example of an effective therapeutic that molecularly targets RXR; this drug was approved to treat cutaneous T cell lymphoma and is still used as a standard of care treatment for this disease. No additional RXR agonists have yet achieved FDA approval, but several promising novel compounds are being developed. In this review, we provide an overview of the multiple mechanisms by which RXR signaling regulates inflammation and tumor immunity. We also discuss the potential of RXR-dependent immune cell modulation for the treatment or prevention of cancer and concomitant challenges and opportunities.

The immunomodulatory role of all-trans retinoic acid in tumor microenvironment

Retinoids are essential nutrients for human beings. Among them, all-trans retinoic acid (ATRA), considered one of the most active metabolites, plays important roles in multiple biological processes. ATRA regulates the transcription of target genes by interacting with nuclear receptors bonded to retinoic acid response elements (RAREs). Besides its differentiation-inducing effect in the treatment of acute promyelocytic leukemia and some solid tumor types, its immunoregulatory role in tumor microenvironment (TME) has attracted considerable attention. ATRA not only substantially abrogates the immunosuppressive effect of tumor-infiltrating myeloid-derived suppressor cells but also activates the anti-tumor effect of CD8 + T cells. Notably, the combination of ATRA with other therapeutic approaches, including immune checkpoint inhibitors (ICIs), tumor vaccines, and chemotherapy, has been extensively investigated in a variety of tumor models and clinical trials. In this review, we summarize the current understanding of the role of ATRA in cancer immunology and immunotherapy, dissect the underlying mechanisms of ATRA-mediated activation or differentiation of different types of immune cells, and explore the potential clinical significance of ATRA-based cancer therapy.

Rexinoids Modulate Effector T Cell Expression of Mucosal Homing Markers CCR9 and α4β7 Integrin and Direct Their Migration In Vitro

Altering T cell trafficking to mucosal regions can enhance immune responses towards pathogenic infections and cancers at these sites, leading to better outcomes. All-trans-retinoic acid (ATRA) promotes T cell migration to mucosal surfaces by inducing transcription of the mucosal-homing receptors CCR9 and α4β7 via binding to retinoic acid receptors (RARs), which heterodimerize with retinoid X receptors (RXRs) to function. However, the unstable nature and toxicity of ATRA limit its use as a widespread treatment modality for mucosal diseases. Therefore, identifying alternatives that could reduce or eliminate the use of ATRA are needed. Rexinoids are synthetically derived compounds structurally similar to ATRA. Originally named for their ability to bind RXRs, rexinoids can enhance RAR-mediated gene transcription. Furthermore, rexinoids are more stable than ATRA and possess an improved safety profile, making them attractive candidates for use in clinical settings. Here we show that select novel rexinoids act as ATRA mimics, as they cause increased CCR9 and α4β7 expression and enhanced migration to the CCR9 ligand, CCL25 in vitro, even in the absence of ATRA. Conversely, other rexinoids act synergistically with ATRA, as culturing cells with suboptimal doses of both compounds resulted in CCR9 expression and migration to CCL25. Overall, our findings show that rexinoids can be used independently or synergistically with ATRA to promote mucosal homing of T cells in vitro, and lends support for the prospective clinical use of these compounds in immunotherapeutic approaches for pathogenic infections or cancers at mucosal surfaces.

Inflammation- and Gut-Homing Macrophages, Engineered to De Novo Overexpress Active Vitamin D, Promoted the Regenerative Function of Intestinal Stem Cells

Inflammatory bowel disease (IBD) is a chronic inflammatory disease of the gut. Available drugs aim to suppress gut inflammation. These drugs have significantly delayed disease progression and improved patients’ quality of life. However, the disease continues to progress, underscoring the need to develop novel therapies. Aside from chronic gut inflammation, IBD patients also experience a leaky gut problem due to damage to the intestinal epithelial layer. In this regard, epithelial regeneration and repair are mediated by intestinal stem cells. However, no therapies are available to directly enhance the intestinal stem cells’ regenerative and repair function. Recently, it was shown that active vitamin D, i.e., 1,25-dihydroxyvitamin D or 1,25(OH)₂D, was necessary to maintain Lgr5⁺ intestinal stem cells, actively cycling under physiological conditions. In this study, we used two strategies to investigate the role of 1,25(OH)₂D in intestinal stem cells’ regenerative function. First, to avoid the side effects of systemic high 1,25(OH)₂D conditions, we used our recently developed novel strategy to deliver locally high 1,25(OH)₂D concentrations specifically to inflamed intestines. Second, because of the Lgr5⁺ intestinal stem cells’ active cycling status, we used a pulse-and-chase strategy via 5-bromo-2′-deoxyuridine (BrdU) labeling to trace the Lgr5⁺ stem cells through the whole epithelial regeneration process. Our data showed that locally high 1,25(OH)₂D concentrations enhanced intestinal stem cell migration. Additionally, the migrated cells differentiated into mature epithelial cells. Our data, therefore, suggest that local delivery of high 1,25(OH)₂D concentrations is a promising strategy to augment intestinal epithelial repair in IBD patients.

Vitamin A controls the allergic response through T follicular helper cell as well as plasmablast differentiation

BACKGROUND

Vitamin A regulates the adaptive immune response and a modulatory impact on type I allergy is discussed. The cellular mechanisms are largely unknown.

OBJECTIVE

To determine the vitamin A-responding specific lymphocyte reaction in vivo.

METHODS

Antigen-specific B and T lymphocytes were analyzed in an adoptive transfer airway inflammation mouse model in response to 9-cis retinoic acid (9cRA) and after lymphocyte-specific genetic targeting of the receptor RARα. Flow cytometry, quantitative PCR, next-generation sequencing, and specific Ig-ELISA were used to characterize the cells functionally.

RESULTS

Systemic 9cRA profoundly enhanced the specific IgA-secreting B-cell frequencies in the lung tissue and serum IgA while reducing serum IgE concentrations. RARα overexpression in antigen-specific B cells promoted differentiation into plasmablasts at the expense of germinal center B cells. In antigen-specific T cells, RARα strongly promoted the differentiation of T follicular helper cells followed by an enhanced germinal center response.

CONCLUSIONS

9cRA signaling via RARα impacts the allergen-specific immunoglobulin response directly by the differentiation of B cells and indirectly by promoting T follicular helper cells.

Potential therapeutic uses of rexinoids

The discovery of nuclear receptors, particularly retinoid X receptors (RXR), and their involvement in numerous pathways related to development sparked interest in their immunomodulatory properties. Genetic models using deletion or overexpression of RXR and the subsequent development of several small molecules that are agonists or antagonists of this receptor support a promising therapeutic role for these receptors in immunology. Bexarotene was approved in 1999 for the treatment of cutaneous T cell lymphoma. Several other small molecule RXR agonists have since been synthesized with limited preclinical development, but none have yet achieved FDA approval. Cancer treatment has recently been revolutionized with the introduction of immune checkpoint inhibitors, but their success has been restricted to a minority of patients. This review showcases the emerging immunomodulatory effects of RXR and the potential of small molecules that target this receptor as therapies for cancer and other diseases. Here we describe the essential roles that RXR and partner receptors play in T cells, dendritic cells, macrophages and epithelial cells, especially within the tumor microenvironment. Most of these effects are site and cancer type dependent but skew immune cells toward an anti-inflammatory and anti-tumor effect. This beneficial effect on immune cells supports the promise of combining rexinoids with approved checkpoint blockade therapies in order to enhance efficacy of the latter and to delay or potentially eliminate drug resistance. The data compiled in this review strongly suggest that targeting RXR nuclear receptors is a promising new avenue in immunomodulation for cancer and other chronic inflammatory diseases.

Nutritional Modulation of Immune and Central Nervous System Homeostasis: The Role of Diet in Development of Neuroinflammation and Neurological Disease

The gut-microbiome-brain axis is now recognized as an essential part in the regulation of systemic metabolism and homeostasis. Accumulating evidence has demonstrated that dietary patterns can influence the development of metabolic alterations and inflammation through the effects of nutrients on a multitude of variables, including microbiome composition, release of microbial products, gastrointestinal signaling molecules, and neurotransmitters. These signaling molecules are, in turn, implicated in the regulation of the immune system, either promoting or inhibiting the production of pro-inflammatory cytokines and the expansion of specific leukocyte subpopulations, such as Th17 and Treg cells, which are relevant in the development of neuroinflammatory and neurodegenerative conditions. Metabolic diseases, like obesity and type 2 diabetes mellitus, are related to inadequate dietary patterns and promote variations in the aforementioned signaling pathways in patients with these conditions, which have been linked to alterations in neurological functions and mental health. Thus, maintenance of adequate dietary patterns should be an essential component of any strategy aiming to prevent neurological pathologies derived from systemic metabolic alterations. The present review summarizes current knowledge on the role of nutrition in the modulation of the immune system and its impact in the development of neuroinflammation and neurological disease.

Single and combined effect of retinoic acid and rapamycin modulate the generation, activity and homing potential of induced human regulatory T cells

Adoptive transfer of CD4+CD25+FOXP3+ regulatory T cells (Treg cells) has been successfully utilized to treat graft versus host disease and represents a promising strategy for the treatment of autoimmune diseases and transplant rejection. The aim of this study was to evaluate the effects of all-trans retinoic acid (atRA) and rapamycin (RAPA) on the number, phenotype, homing markers expression, DNA methylation, and function of induced human Treg cells in short-term cultures. Naive T cells were polyclonally stimulated and cultured for five days in the presence of different combinations of IL-2, TGF-β1, atRA and RAPA. The resulting cells were characterized by the expression of FOXP3, activation, surface and homing markers. Methylation of the Conserved Non-coding Sequence 2 was also evaluated. Functional comparison of the different culture conditions was performed by suppression assays in vitro. Culturing naive human T cells with IL-2/TGFβ1 resulted in the generation of 54.2% of Treg cells (CD4+CD25+FOXP3+) whereas the addition of 100 nM atRA increased the yield of Treg cells to 66% (p = 0.0088). The addition of RAPA did not increase the number of Treg cells in any of these settings. Treg cells generated in the presence of atRA had an increased expression of the β7 integrin to nearly 100% of the generated Treg cells, while RAPA treated cells showed enhanced expression of CXCR4. The differential expression of homing molecules highlights the possibility of inducing Treg cells with differential organ-specific homing properties. Neither atRA nor RAPA had an effect on the highly methylated CNS2 sites, supporting reports that their contribution to the lineage stability of Treg cells is not mediated by methylation changes in this locus. Treg cells generated in the presence of RAPA show the most potent suppression effect on the proliferation of effector cells.

Impaired TNF/TNFR2 signaling enhances Th2 and Th17 polarization and aggravates allergic airway inflammation

CD4⁺ T-cell differentiation plays an important role in allergic airway diseases. Tumor necrosis factor receptor 2 (TNFR2) has been shown to regulate CD4⁺ T-lymphocyte differentiation, but its role in allergic airway inflammation is not clear. Here, we investigated the role of TNFR2 in allergic airway inflammation. The mouse model was generated by immunization with ovalbumin and intranasal administration of TNFR2 antibody. Airway inflammation and CD4⁺ T-cell differentiation were measured in vivo and in vitro. Inhibited TNFR2 signaling aggravated airway inflammation and increased the expression of inflammatory cytokines (IL-4, IL-5, IL-17, and TNF-α) in serum and bronchoalveolar lavage fluid. Impaired TNFR2 signaling promoted Th2 and Th17 polarization but inhibited Th1 and CD4⁺CD25⁺ T-cell differentiation in vivo. Furthermore, TNFR2 signaling inhibition promoted Th2 and Th17 polarization in vitro, which may occur through the activation of TNF receptor-associated factor 2 and NF-κB signaling. Therefore, our findings indicate that impaired TNF/TNFR2 signaling enhances Th2 and Th17 polarization and aggravates allergic airway inflammation.

mTOR is critical for intestinal T-cell homeostasis and resistance to Citrobacter rodentium

T-cells play an important role in promoting mucosal immunity against pathogens, but the mechanistic basis for their homeostasis in the intestine is still poorly understood. We report here that T-cell-specific deletion of mTOR results in dramatically decreased CD4 and CD8 T-cell numbers in the lamina propria of both small and large intestines under both steady-state and inflammatory conditions. These defects result in defective host resistance against a murine enteropathogen, Citrobacter rodentium, leading to the death of the animals. We further demonstrated that mTOR deficiency reduces the generation of gut-homing effector T-cells in both mesenteric lymph nodes and Peyer’s patches without obviously affecting expression of gut-homing molecules on those effector T-cells. Using mice with T-cell-specific ablation of Raptor/mTORC1 or Rictor/mTORC2, we revealed that both mTORC1 and, to a lesser extent, mTORC2 contribute to both CD4 and CD8 T-cell accumulation in the gastrointestinal (GI) tract. Additionally, mTORC1 but not mTORC2 plays an important role regulating the proliferative renewal of both CD4 and CD8 T-cells in the intestines. Our data thus reveal that mTOR is crucial for T-cell accumulation in the GI tract and for establishing local adaptive immunity against pathogens.

A Novel mTORC1-Dependent, Akt-Independent Pathway Differentiates the Gut Tropism of Regulatory and Conventional CD4 T Cells

The vitamin A metabolite all-trans retinoic acid (ATRA) induces a gut-homing phenotype in activated CD4(+) conventional T cells (Tconv) by upregulating the integrin α4β7 and the chemokine receptor CCR9. We report that, in contrast to mouse Tconv, only ∼50% of regulatory T cells (Treg) upregulate CCR9 when stimulated by physiological levels of ATRA, even though Tconv and Treg express similar levels of the retinoic acid receptor (RAR). The resulting bimodal CCR9 expression is not associated with differences in the extent of their proliferation, level of Foxp3 expression, or affiliation with naturally occurring Treg or induced Treg in the circulating Treg pool. Furthermore, we find that exposure of Treg to the mechanistic target of rapamycin (mTOR) inhibitor rapamycin suppresses upregulation of both CCR9 and α4β7, an effect that is not evident with Tconv. This suggests that in Treg, ATRA-induced upregulation of CCR9 and α4β7 is dependent on activation of a mTOR signaling pathway. The involvement of mTOR is independent of Akt activity, because specific inhibition of Akt, pyruvate dehydrogenase kinase-1, or its downstream target glycogen synthase kinase-3 did not prevent CCR9 expression. Additionally, Rictor (mTOR complex [mTORC]2)-deficient Treg showed unaltered ability to express CCR9, whereas Raptor (mTORC1)-deficient Treg were unable to upregulate CCR9, suggesting the selective participation of mTORC1. These findings reveal a novel difference between ATRA signaling and chemokine receptor induction in Treg versus Tconv and provide a framework via which the migratory behavior of Treg versus Tconv might be regulated differentially for therapeutic purposes.

Immunity and Tolerance Induced by Intestinal Mucosal Dendritic Cells

Dendritic cells present in the digestive tract are constantly exposed to environmental antigens, commensal flora, and invading pathogens. Under steady-state conditions, these cells have high tolerogenic potential, triggering differentiation of regulatory T cells to protect the host from unwanted proinflammatory immune responses to innocuous antigens or commensals. On the other hand, these cells must discriminate between commensal flora and invading pathogens and mount powerful immune response against pathogens. A potential result of unbalanced tolerogenic versus proinflammatory responses mediated by dendritic cells is associated with chronic inflammatory conditions, such as Crohn's disease, ulcerative colitis, food allergies, and celiac disease. Herein, we review the dendritic cell population involved in mediating tolerance and immunity in mucosal surfaces, the progress in unveiling their development in vivo, and factors that can influence their functions.

Intestinal mucosal tolerance and impact of gut microbiota to mucosal tolerance

The mucosal barriers are very sensitive to pathogenic infection, thereby assuming the capacity of the mucosal immune system to induce protective immunity to harmful antigens and tolerance against harmless substances. This review provides current information about mechanisms of induction of mucosal tolerance and about impact of gut microbiota to mucosal tolerance.

Retinoids induce integrin-independent lymphocyte adhesion through RAR-α nuclear receptor activity

Oxidative metabolites of vitamin A, in particular all-trans-retinoic acid (atRA), have emerged as key factors in immunity by specifying the localization of immune cells to the gut. Although it is appreciated that isomers of retinoic acid activate the retinoic acid receptor (RAR) and retinoid X receptor (RXR) family of nuclear receptors to elicit cellular changes, the molecular details of retinoic acid action remain poorly defined in immune processes. Here we employ a battery of agonists and antagonists to delineate the specific nuclear receptors utilized by retinoids to evoke lymphocyte cell adhesion to ADAM (adisintegrin and metalloprotease) protein family members. We report that RAR agonism is sufficient to promote immune cell adhesion in both immortal and primary immune cells. Interestingly, adhesion occurs independent of integrin function, and mutant studies demonstrate that atRA-induced adhesion to ADAM members required a distinct binding interface(s) as compared to integrin recognition. Anti-inflammatory corticosteroids as well as 1,25-(OH)2D3, a vitamin D metabolite that prompts immune cell trafficking to the skin, potently inhibited the observed adhesion. Finally, our data establish that induced adhesion was specifically attributable to the RAR-α receptor isotype. The current study provides novel molecular resolution as to which nuclear receptors transduce retinoid exposure into immune cell adhesion.

Organ-specific and memory treg cells: specificity, development, function, and maintenance

Foxp3⁺ regulatory T cells (Treg cells) are essential for establishing and maintaining self-tolerance, and also inhibit immune responses to innocuous environmental antigens. Imbalances and dysfunction in Treg cells lead to a variety of immune-mediated diseases, as deficits in Treg cell function contribute to the development autoimmune disease and pathological tissue damage, whereas overabundance of Treg cells can promote chronic infection and tumorigenesis. Recent studies have highlighted the fact that Treg cells themselves are a diverse collection of phenotypically and functionally specialized populations, with distinct developmental origins, antigen-specificities, tissue-tropisms, and homeostatic requirements. The signals directing the differentiation of these populations, their specificities and the mechanisms by which they combine to promote organ-specific and systemic tolerance, and how they embody the emerging property of regulatory memory are the focus of this review.

Retinoic acid prevents mesenteric lymph node dendritic cells from inducing IL-13-producing inflammatory Th2 cells

The vitamin A (VA) metabolite retinoic acid (RA) affects the properties of T cells and dendritic cells (DCs). In VA-deficient mice, we observed that mesenteric lymph node (MLN)-DCs induce a distinct inflammatory T helper type 2 (Th2)-cell subset that particularly produces high levels of interleukin (IL)-13 and tumor necrosis factor-α (TNF-α). This subset expressed homing receptors for skin and inflammatory sites, and was mainly induced by B220(-)CD8α(-)CD11b(+)CD103(-) MLN-DCs in an IL-6- and OX40 ligand-dependent manner, whereas RA inhibited this induction. The corresponding MLN-DC subset of VA-sufficient mice induced a similar T-cell subset in the presence of RA receptor antagonists. IL-6 induced this subset differentiation from naive CD4(+) T cells upon activation with antibodies against CD3 and CD28. Transforming growth factor-β inhibited this induction, and reciprocally enhanced Th17 induction. Treatment with an agonistic anti-OX40 antibody and normal MLN-DCs enhanced the induction of general inflammatory Th2 cells. In VA-deficient mice, proximal colon epithelial cells produced TNF-α that may have enhanced OX40 ligand expression in MLN-DCs. The repeated oral administrations of a T cell-dependent antigen primed VA-deficient mice for IL-13-dependent strong immunoglobulin G1 (IgG1) responses and IgE responses that caused skin allergy. These results suggest that RA inhibits allergic responses to oral antigens by preventing MLN-DCs from inducing IL-13-producing inflammatory Th2 cells.

Modulation of T cell and innate immune responses by retinoic Acid

Retinoic acid (RA) is produced by a number of cell types, including macrophages and dendritic cells, which express retinal dehydrogenases that convert vitamin A to its main biologically active metabolite, all-trans RA. All-trans RA binds to its nuclear retinoic acid receptors that are expressed in lymphoid cells and act as transcription factors to regulate cell homing and differentiation. RA production by CD103(+) dendritic cells and alveolar macrophages functions with TGF-β to promote conversion of naive T cells into Foxp3(+) regulatory T cells and, thereby, maintain mucosal tolerance. Furthermore, RA inhibits the differentiation of naive T cells into Th17 cells. However, Th1 and Th17 responses are constrained during vitamin A deficiency and in nuclear RA receptor α-defective mice. Furthermore, RA promotes effector T cell responses during infection or autoimmune diseases. Thus, RA plays a role in immune homeostasis in the steady-state but activates pathogenic T cells in conditions of inflammation.

Attenuation of migration properties of CD4+ T cells from aged mice correlates with decrease in chemokine receptor expression, response to retinoic acid, and RALDH expression compared to young mice

Aging results in attenuation of abilities to mount appropriate immune responses. The influence of aging on CD4+ T cell migration ability toward chemokines was investigated with young and aged mice. We found functional decline in migration ability toward CCL19 and also decreased CCR7 expression level in antigen-stimulated CD4+ T cells from aged mice compared with those from young mice. Upon addition of retinoic acid (RA), CD4+ T cells from aged mice showed decreased CCR9 expression level compared to young mice and the migration ability of CD4+ T cells from aged mice toward CCL25 was attenuated compared to young mice. We also observed that the expression of RALDH2 mRNA was decreased in mesenteric lymph node dendritic cells from aged mice compared to those from young mice. These results demonstrate that attenuated migration abilities of CD4+ T cells were observed in aged mice, which correlated with decreased chemokine receptor expression. Furthermore, the reduced production and response to RA by aging may be one of the causes of such attenuated migration abilities in the intestinal immune system.

Retinoid X receptor agonists modulate Foxp3⁺ regulatory T cell and Th17 cell differentiation with differential dependence on retinoic acid receptor activation

Retinoic acid (RA) enhances TGF-β-dependent differentiation of Foxp3(+) inducible regulatory T cells (iTregs) and inhibits Th17 differentiation by binding to the RA receptor (RAR)/retinoid X receptor (RXR) heterodimer. The major physiologic RA, all-trans-RA, binds to RAR but not to RXR at physiological concentrations. It remained unclear whether RXR-mediated stimulation affected the iTregs and Th17 differentiation. We found in this study that the RXR agonists, PA024 and tributyltin, augmented the ability of all-trans-RA or the RAR agonist Am80 to enhance CD4(+)CD25(-) T cells to acquire Foxp3 expression and suppressive function. However, they failed to enhance Foxp3 expression in the presence of the RAR antagonist LE540, suggesting that the effect depends on RAR-mediated signals. They exerted the effect largely by augmenting the ability of all-trans-RA to suppress the production of IL-4, IL-21, and IFN-γ that inhibited Foxp3 expression. Agonists of peroxisome proliferator-activated receptors and liver X receptors (LXRs), permissive partners of RXR, failed to enhance Foxp3 expression. In contrast, RXR agonists and LXR agonists suppressed IL-17 expression. The RXR-mediated suppression was not canceled by blocking RAR stimulation but was likely to involve permissive activation of LXRs. All-trans-RA and an agonist of RXR or LXR additively suppressed IL-17 expression when the all-trans-RA concentration was low. RXR agonists also suppressed Ccr6 expression that is essential for Th17 cells to enter the CNS. Accordingly, tributyltin treatment of mice ameliorated experimental autoimmune encephalomyelitis through regulating Th17 cell activities. These results suggest that RXR stimulation modulates Foxp3(+) iTreg and Th17 differentiation with differential dependence on RAR-mediated stimulation.

Host and microbial factors in regulation of T cells in the intestine

The intestine is divided into specialized tissue areas that provide distinct microenvironments for T cells. Regulation of T-cell responses in the gut has been a major focus of recent research activities in the field. T cells in the intestine are regulated by the interplay between host and microbial factors. In the small intestine, retinoic acid (RA) is a major tissue factor that plays important roles in regulation of immune responses. In the large intestine, the influence of RA diminishes, but that of commensal bacterial products increases. RA, gut microbiota, and inflammatory mediators co-regulate differentiation, distribution, and/or effector functions of T cells. Coordinated regulation of immune responses by these factors promotes well-balanced immunity and immune tolerance. Dysregulation of this process can increase infection and inflammatory diseases.

Ligand binding pocket function of Drosophila USP is necessary for metamorphosis

The widely accepted paradigm that epoxidized methyl farnesoates ("juvenile hormones," JHs) are the principal sesquiterpenoid hormones regulating insect metamorphosis was assessed in Drosophila melanogaster. GC-MS analysis of circulating methyl farnesoids during the mid to late 3rd instar showed that methyl farnesoate is predominant over methyl epoxyfarnesoate (=JH III). The circulating concentration of methyl farnesoate (reaching nearly 500 nM), was easily high enough on a kinetic basis to load the Drosophila ortholog of the nuclear hormone receptor RXR (also known as "ultraspiracle," USP), whereas the circulating concentrations of JH III and methyl bisepoxyfarnesoate (bisepoxyJH III) were not. The hypothesis that the ligand pocket of USP necessarily binds an endogenous ligand for differentiation of the immature to the adult was tested with USP mutated at residue that normally extends a side chain into the ligand binding pocket. An equilibrium binding assay confirmed that the mutation (Q288A) strongly altered methyl farnesoate interaction with USP, while a heterologous cell-line transfection assay confirmed that the mutation did not allosterically alter the transcriptional response of the ultraspiracle/ecdysone receptor heterodimer to ecdysteroid signaling. Transgenic wildtype USP driven by the cognate natural promoter rescued null animals to develop to the adult inside a normally formed puparium, while in contrast animals transgenically expressing instead the ligand pocket mutant exhibited developmental derangement at the larval to pupal transition, including failure to form a properly shaped or sclerotized puparium. Other point mutations to the pocket strongly reducing affinity for methyl farnesoate similarly disrupted the larval to pupal metamorphosis. These results suggest that normal larval to pupal maturation in this mecopteran model insect requires the involvement of a distinct endocrine axis of USP binding to its own endogenous terpenoid ligand.

TNFR2 on non-haematopoietic cells is required for Foxp3+ Treg-cell function and disease suppression in EAE

The TNF/TNFR system exerts multiple proinflammatory and immunosuppressive functions in the pathogenesis of chronic inflammation and autoimmunity. In EAE, the experimental model of Multiple Sclerosis (MS), genetic ablation of TNFR2, results in exacerbated immune reactivity and chronic disease course. The underlying mechanism driving this immunosuppressive function of TNFR2 remains unclear. We show here that chronic exacerbated EAE in TNFR2 KO mice is associated with increased Th17-cell responses and reduced numbers of Foxp3(+) Treg cells both in the spinal cord and peripheral lymphoid organs. Treg cells from TNFR2-deficient animals developing EAE show decreased proliferative and suppressive functions, both ex vivo and in vivo, and appear responsible for the exacerbated non-remitting disease, as evidenced by phenotypic rescue following adoptive transfer of Treg cells from WT but not TNFR2(-/-) donors. Reciprocal BM transplantation experiments between WT and TNFR2-deficient mice demonstrated that the capacity of TNFR2 to support Treg-cell expansion and function during EAE is non-intrinsic to Treg or other haematopoietic cells but requires expression of TNFR2 in radiation-resistant cells of the host. These results reveal a previously unsuspected role for non-haematopoietic TNFR2 in modulating Treg-cell expansion and immune suppression during development of autoimmunity and suggest that a similar mechanism may affect chronicity and relapses characterizing human autoimmune disease, including MS.

Cyp26b1 regulates retinoic acid-dependent signals in T cells and its expression is inhibited by transforming growth factor-β

Background

The vitamin A metabolite, retinoic acid (RA), plays important roles in the regulation of lymphocyte properties. Dendritic cells in gut-related lymphoid organs can produce RA, thereby imprinting gut-homing specificity on T cells and enhancing transforming growth factor (TGF)-β-dependent induction of Foxp3⁺ regulatory T cells upon antigen presentation. In general, RA concentrations in cells and tissues are regulated by its degradation as well. However, it remained unclear if T cells could actively catabolize RA.

Methodology/Principal Findings

We assessed the expression of known RA-catabolizing enzymes in T cells from mouse lymphoid tissues. Antigen-experienced CD44⁺ T cells in gut-related lymphoid organs selectively expressed Cyp26b1, a member of the cytochrome P450 family 26. However, T cells in the spleen or skin-draining lymph nodes did not significantly express Cyp26b1. Accordingly, physiological levels of RA (1–10 nM) could induce Cyp26b1 expression in naïve T cells upon activation in vitro, but could not do so in the presence of TGF-β. Overexpression of Cyp26b1 significantly suppressed the RA effect to induce expression of the gut-homing receptor CCR9 on T cells. On the other hand, knocking down Cyp26b1 gene expression with small interfering RNA or inhibiting CYP26 enzymatic activity led to enhancement of the RA-induced CCR9 expression.

Conclusions/Significance

Our data demonstrate a role for CYP26B1 in regulating RA-dependent signals in activated T cells but not during TGF-β-dependent differentiation to Foxp3⁺ regulatory T cells. Aberrant expression of CYP26B1 may disturb T cell trafficking and differentiation in the gut and its related lymphoid organs.

Retinoic acid production by intestinal dendritic cells

Subpopulations of dendritic cells (DCs) in the small intestine and its related lymphoid organs can produce retinoic acid (RA) from vitamin A (retinol). Through the RA production, these DCs play a pivotal role in imprinting lymphocytes with gut-homing specificity, and contribute to the development of immune tolerance by enhancing the differentiation of Foxp3(+) regulatory T cells and inhibiting that of inflammatory Th17 cells. The RA-producing capacity in these DCs mostly depends on the expression of retinal dehydrogenase 2 (RALDH2, ALDH1A2). It is likely that the RALDH2 expression is induced in DCs by the microenvironmental factors in the small intestine and its related lymphoid organs. The major factor responsible for the RALDH2 expression appears to be GM-CSF. RA itself is essential for the GM-CSF-induced RALDH2 expression. IL-4 and IL-13 also enhance RALDH2 expression, but are dispensable. Toll-like receptor-mediated signals can also enhance the GM-CSF-induced RALDH2 expression in immature DCs.

Retinoic acid-induced CCR9 expression requires transient TCR stimulation and cooperativity between NFATc2 and the retinoic acid receptor/retinoid X receptor complex

Retinoic acid (RA) imprints gut-homing specificity on T cells upon activation by inducing the expression of chemokine receptor CCR9 and integrin α4β7. CCR9 expression seemed to be more highly dependent on RA than was the α4β7 expression, but its molecular mechanism remained unclear. In this article, we show that NFAT isoforms NFATc1 and NFATc2 directly interact with RA receptor (RAR) and retinoid X receptor (RXR) but play differential roles in RA-induced CCR9 expression on murine naive CD4(+) T cells. TCR stimulation for 6-24 h was required for the acquisition of responsiveness to RA and induced activation of NFATc1 and NFATc2. However, RA failed to induce CCR9 expression as long as TCR stimulation continued. After terminating TCR stimulation or adding cyclosporin A to the culture, Ccr9 gene transcription was induced, accompanied by inactivation of NFATc1 and sustained activation of NFATc2. Reporter and DNA-affinity precipitation assays demonstrated that the binding of NFATc2 to two NFAT-binding sites and that of the RAR/RXR complex to an RA response element half-site in the 5'-flanking region of the mouse Ccr9 gene were critical for RA-induced promoter activity. NFATc2 directly bound to RARα and RXRα, and it enhanced the binding of RARα to the RA response element half-site. NFATc1 also bound to the NFAT-binding sites and directly to RARα and RXRα, but it inhibited the NFATc2-dependent promoter activity. These results suggest that the cooperativity between NFATc2 and the RAR/RXR complex is essential for CCR9 expression on T cells and that NFATc1 interferes with the action of NFATc2.